Recently worldwide environmental disruption has become a serious problem. In particular, combustion of fossil fuels, such as petroleums and coals, is accompanied by the generation of nitrogen oxides (NO.sub.x) and sulfur oxides (SO.sub.x), which when released into atmosphere, make acidic rain or acidic fog, to damage severely the natural environment of forests, lakes and mashes. Also, SO.sub.x and NO.sub.x, and further the particulates (such as the particulates of soot, dust and mist) released with the combustion exhaust are harmful to human bodies inhaling them. These circumstances demand some measure for as much reduction of the release of these pollutants including SO.sub.x, NO.sub.x and the particulates as possible.
A common measure for reducing the SO.sub.x release is an after-treatment following the combustion, i.e. flue gas desulfurization, which however is impractical for moving exhaust sources even if operable technically. It is therefore desirable to reduce the sulfur content; in fuels to a degree which no longer requires the flue gas desulfurization.
On the other hand. NO.sub.x occur necessarily even from well denitrified fuels when air is used for combustion, and particulates also occur in a large quantity depending on the kinds of fuels and the manners of combustion, thereby necessitating exhaust gas treatments for inhibiting release of NO.sub.x and particulates into atmosphere.
In the cases of stationary exhaust sources, such as boilers, it has become possible to remove fairly well the atmospheric pollutants, such as SO.sub.x, NO.sub.x and the particulates, by flue gas desulfurization or NO.sub.x removal. In the case of gasoline engines, such as those of passenger cars, the simultaneous removal of NO.sub.x, CO and hydrocarbons from exhaust gas has been effected by a ternary catalyst system, with the SO.sub.x content in the exhaust gas maintained low by using gasoline with low sulfur content. On the other hand, in the cases of diesel engines of trucks or the like, which are moving exhaust sources of these pollutants, the exhaust contains large quantities of NO.sub.x and the particulates including soot, which are now released as they are into atmosphere because of the absence of completed purification techniques. In such circumstances, there arises quickly the development of techniques for removing particulates also from diesel exhaust by oxidation or trapping and for removing NO.sub.x by reduction with catalysts operable in the atmosphere of oxygen. However, diesel engines use gas oils or the like as fuels and release exhaust gas containing a considerable concentration of SO.sub.x due to the present insufficient reduction of the sulfur content in the fuels. The SO.sub.x not only cause atmospheric pollution but also hinder considerably the removal of NO.sub.x from the exhaust. In order to remove NO.sub.x from the exhaust of diesel engines, it is necessary to treat the exhaust in after-treatment apparatuses with NO.sub.x -removal catalysts. During such after-treatments, if the reduction of the concentration of the coexisting SO.sub.x is insufficient, the remaining SO.sub.x considerably accelerate the poisoning of the catalysts, and make it difficult to stably maintain sufficient catalytic functions for a long term. That is, a sufficient reduction of the sulfur content in diesel fuels, such as gas oils, is necessary also from the viewpoint of the maintenance of the catalytic functions.
Thus there is an urgent demand for more effective techniques for reducing the sulfur content in various hydrocarbon fuels, particularly in diesel fuels, such as gas oils.
Such a demand is not limited in fuel fields. The sulfur (or organic nitrogen compounds in some cases) contained in hydrocarbons generally deteriorates the catalytic functions in many catalytic processes, such as catalytic cracking or catalytic reforming processes. This also causes a strong demand for improved techniques for the effective removal of the sulfur compounds or organic nitrogen compounds from material hydrocarbons, particularly for the development of more efficient hydrodesulfurization catalysts.
There have been provided many techniques for the hydrodesulfurization of sulfur-containing hydrocarbons. In these conventional techniques were proposed various catalysts having the activity of hydrodesulfurizing gas oil fractions or the like, among which that are typical and widely used are those resulting from various modifications of alumina-base catalysts loaded with Group VIA metals and Group VIII metals of the Periodic Table. However, the above-described circumstances have demanded further improvement in the catalytic functions of these conventional hydrodesulfurization catalysts.
The functions of such metal-loaded catalysts are known to depend considerably on the state of loading of the metal component, and the development of the preparation techniques, particularly the technique for loading the metal component, seems to be very important. As to the above-described hydrodesulfurization catalysts, there have already been proposed various preparation techniques, and typical one is so-called impregnation technique, wherein an alumina support prepared by calcining alumina gel is loaded with salts of metals, such as Group VIA metals and Group VIII metals, by impregnation. However, in the conventional impregnation technique, namely the loading technique by impregnating calcined alumina support with aqueous metal salt solutions, when it is attempted to increase the loading rate of the active metal component (effectively loaded component consisting of Group VIA metals; and Group VIII metals) to improve the desulfurization activity, there arises difficulty in loading, or, even if the loading rate could be increased, the supported active metal component aggregates easily to lower their dispersion, thereby making the improvement in activity insufficient or, in some cases, rather reducing the catalytic functions, including activities.
It has been proposed to improve the impregnation techniques by adding additives, such as oxy acids (hydroxycarboxylic acids) or EDTA, to the impregnation liquid [Japanese Patent Application Kokoku Koho (publication) No. 4-15-33940, Japanese Patent Application Kokoku Koho (publication) No. 55-33940, Japanese Patent Application Kokai Koho (Laid-open) No. 60-187337, etc.]. These techniques are advantageous in that the additives improve the stability of the aqueous metal salt solutions, so that the aggregation of the loaded metal component can be prevented remarkably by, though troublesome, selecting the optimum impregnation conditions, as compared with the conventional impregnation, technique. However, due to the fundamental impregnation technique as described above, even these improved techniques involve the problem that increasing the metal component also causes easy aggregation of the loaded metal component, failing in giving highly active catalysts.
Another known technique is so-called kneading technique, wherein catalysts are prepared by kneading alumina gel (hydrogel) per se with metal salts (aqueous solution), followed by calcination [Japanese Patent Application Kokai Koho (Laid-open) No. 1-164440, Japanese Patent Application Kokoku Koho (Publication) No. 53-6113]. According to the kneading technique, generally, the loading rate of the metal component can be increased easily as compared with the above-described impregnation techniques. However, in such a conventional kneading technique, a highly concentrated alumina gel is allowed to .contact metal salts, so that the contact occurs only partially (locally) not to effect microscopically effective mixing. This results in uneven loading, and particularly the metal component is apt to be maldistributed or to remain separated from the alumina support and, at the time of calcination or pretreatments, changes to inconvenient form (for example, metal particles of low distribution or isolated metal particles), which affect adversely the catalytic functions, for example, decreases hydrodesulfurization activities or causes excessive hydrocracking. There is a report that the catalysts prepared by such a conventional kneading technique are generally inferior in hydrodesulfurization activities to the catalysts prepared by the above-described impregnation techniques [refer to Kogyo Kagaku Zasshi, 74, No. 3, page 10 (1971)].
Also proposed is a combination of kneading and impregnation [Japanese Patent Application Kokai Koho (Laid-open) Nos. 61-138537 and 51-24593]. This technique needs very complicated operation, which causes problems, such as the high production cost.